The present invention relates, in general, to analog amplifiers and, more particularly, to Radio Frequency (RF) downconverters or mixers.
Receiver circuits are used in a myriad of wireless communication applications such as cordless telephones, pagers, cable modems, and cellular telephones. A receiver circuit typically receives a Radio Frequency (RF) modulated signal from an antenna. The receiver includes an input low-noise amplifier followed by a filter and a mixer. The RF signals are transmitted at a high frequency and are received by the mixer. The mixer uses a local oscillator signal for downconverting the RF signal to an IF signal for additional processing. It is desirable to maintain the receiver sensitivity, linearity, and noise figure over a wide range of input signal levels and input frequencies.
RF mixers include a transconductor and a commutating stage switching section driven by signals from a local oscillator.
The transconductance of the RF mixer is set by the input impedance, which typically has a value of fifty ohms for RF circuits. The gain of the RF mixer is the product of the output impedance and the transconductance. Thus, the voltage gain of the RF mixer is limited by the transconductance value and the gain of the RF mixer is increased by increasing the output load impedance. However, the output load impedance is usually derived from a tuned circuit, which places a practical limit on the value of the output load impedance. When the output impedance is increased to provide higher gain, the Q of the tuned circuit is increased which causes the bandwidth of the RF mixer to be lowered.
Hence, a need exists for an RF mixer that has a wide band of operation, low noise, high gain, and high linearity while maintaining low current consumption when downconverting for dual-band RF applications.